This invention relates generally to broadband communications systems, such as cable television networks, and more specifically to a carrier-detect device that detects the presence of a carrier signal that is transmitted in the reverse path of the broadband communications system.
FIG. 1 is a block diagram illustrating an example of one branch of a conventional broadband communications system, such as a two-way hybrid/fiber coaxial (HFC) network, that carries optical and electrical signals. Such a network may be used in a variety of systems, including, for example, cable television networks, voice delivery networks, and data delivery networks to name but a few. The communications system 100 includes headend equipment 105 for generating forward, or downstream, signals (e.g., voice, audio, video, or data signals) that are transmitted to subscriber equipment 145. Initially, the forward signals are transmitted as optical signals along a first communication medium 110, such as a fiber optic cable. In most networks, the first communication medium 110 is a long haul segment that carries light having a wavelength in the 1550 nanometer (nm) range. The first communication medium 110 carries the forward signal to hubs 115, which include equipment that transmits the optical signals over a second communication medium 120. In most networks, the second communication medium 120 is an optical fiber that is designed for shorter distances, and which carries light having a wavelength in the 1310 nm range.
From the hub 115, the signals are transmitted to an optical node 125 that converts the optical signals to radio frequency (RF), or electrical, signals. The electrical signals are then transmitted along a third communication medium 130, such as coaxial cable, and are amplified and split, as necessary, by one or more distribution amplifiers 135a-c positioned along the communication medium 130. Taps 140 further split the forward signals in order to provide signals to subscriber equipment 145, such as set-top terminals, computers, telephone handsets, modems, televisions, etc. It will be appreciated that only one branch of the network connecting the headend equipment 105 with the plurality of subscriber equipment 145 is shown for simplicity. However, those skilled in the art will appreciate that most networks include several different branches connecting the headend equipment 105 with several additional hubs 115, optical nodes 125, amplifiers 135a-c, and subscriber equipment 145.
In a two-way network, the subscriber equipment 145 generates reverse RF signals, which may be generated for a variety of purposes, including e-mail, web surfing, pay-per-view, video-on-demand, telephony, and administrative signals from the set-top terminal. These reverse RF signals are typically in the form of modulated RF carriers that are transmitted upstream through the reverse path to the headend equipment 105. The reverse electrical signals from various subscribers are combined via the taps 140 and passive electrical combiners (not shown) with other reverse signals from other subscriber equipment 145. The combined reverse electrical signals are amplified by one or more of the distribution amplifiers 135a-c and typically converted to optical signals by the optical node 125 before being provided to the headend equipment 105. It will be appreciated that in the electrical, or RF, portion of the network 100, the forward and reverse electrical signals are carried along the same coaxial cable 130. In contrast, the forward and reverse optical signals on the first and second communications media 110, 120 are usually carried on separate optical fibers.
The reverse RF carrier signals are generally transmitted within a frequency range from 5 MHz to, for example, 42 MHz. FIG. 2 illustrates a typical reverse band and the frequencies allocated to various services that may be used by the subscriber equipment 145 for the purpose of sending reverse carrier signals. It will be appreciated that the combined reverse carrier signals may include a plurality of reverse carrier signals from a plurality of subscriber equipment. The combined carrier signals may also include a variety of signals in a plurality of frequencies. In addition to the carrier signals, noise and interference is often present in the system. Typically, the noise signals can be viewed with test equipment as essentially a constant level, or noise floor, that most particularly affects the reverse path signals. Disadvantageously, the noise signals interfere with the processing of the valid carrier signals by the headend equipment.
When necessary, the presence of carrier signals in the reverse path is typically detected among the noise signals by examining the instantaneous power level of the RF signal. For example, if the power level of the RF signal is above a predetermined threshold for a predetermined amount of time, e.g., at least 2 microseconds during any 8-microsecond window, the RF signal is treated as a valid RF carrier signal. Otherwise the reverse signal is treated as just noise and interference. This carrier-detect scheme works adequately for narrowband noise that has a predictable noise floor; however, wideband noise, which has a high peak to average power ratio in the time domain, can exceed the predetermined threshold for 2 microseconds in an 8-microsecond window even though its average power level is low. Disadvantageously, the conventional carrier-detect devices allow the transmission of noise signals more than desired.
The present invention is, therefore, directed to an improved carrier-detect circuit that detects a valid reverse carrier signal while significantly limiting the transmission of undesirable noise signals. As a result, the HFC network""s reverse path signaling capacity, quality, and reliability are greatly enhanced.